1. Field of the Invention
The present invention relates to a light-unit, a backlight, a front light, and a display device. More specifically, the present invention relates to a light unit preferably used in a side-light type backlight or front light, and a display device such as a liquid crystal display device, including the light unit.
2. Description of the Related Art
A display device including a non-self-emitting display panel, such as a liquid crystal display panel, needs to include light sources in order to display an image. Reflected sunlight is used as a light source, but a light unit needs to be arranged in the following display devices: a display device which is mainly used indoors such as a word processor, a laptop personal computer, and an in-car liquid crystal display device (LCD); and a display device which is used outdoors but constantly needs a certain light amount.
Side-light type and direct-type backlights and the like are known as the light unit. A side-light type backlight in a liquid crystal display device including a small-sized screen has been widely used because it can display an image with a small power consumption using a small number of light sources.
Examples of members for the backlight include, for example, a reflective film, a diffusion sheet, and a prism sheet, in addition to the light source. The side-light type backlight has a structure in which light sources are arranged on a lateral edge of a display surface, and therefore the backlight is further provided with a light guide plate, which is different from the direct-type backlight. The light guide plate is generally made of a colorless and transparent material, and it is a plate member with which a pattern for reflecting, refracting, or diffusing light is provided. Furthermore, according to the side-light type backlight, the light guide plate is arranged below the display panel such that the main surface of the light guide plate faces the display screen and such that point light sources or linear light sources are arranged to oppose a side surface of the light guide plate. According to such a configuration, lights from the point light sources or linear light sources enter the light guide plate from the side surface of the light guide plate. Further, the lights are reflected, refracted, or diffused by the pattern provided for the light guide plate when passing through the inside of the light guide plate, and then the lights are emitted into a planar shape from the main surface side.
In the display device, a frontlight may be used instead of the backlight. According to a side-light type frontlight, point light sources or linear light sources are arranged to face a side surface of a light guide plate, similarly in the backlight. Lights from the point light sources or the linear light sources enter the light guide plate from the side surface of the light guide plate. Further, the lights are reflected, refracted, or diffused by the pattern provided for the light guide plate when passing through the inside of the light guide plate, and then the lights are once outputted into a planar shape from the main surface side to the display screen side as output light. In the frontlight, the light guide plate is arranged on the side opposite to the side where the light guide plate is arranged in the backlight relative to a liquid crystal layer. Lights which have passed through the light guide plate are reflected by a reflector such as a reflective sheet, provided on the side opposite to the light guide plate relative to the liquid crystal layer. Therefore, the lights pass through the liquid crystal layer twice.
Recently, the use of a thin display device, such as a liquid crystal display device, has increasingly spread. Along with this, such a display device needs higher display qualities. For the side-light type backlight or frontlight, it is necessary to increase a luminance of light outputted through the light guide plate and further increase uniformity of the luminance.
For such a need, luminance characteristics have conventionally been improved by modifying a structure of the light guide plate or arrangement of light sources (for example, refer to Japanese Kokai Publication No. 2001-43718, Japanese Kokai Publication No. 2002-100231, Japanese Kokai Publication No. 2002-157911, Japanese Kokai Publication No. 2003-132722, Japanese Kokai Publication No. 2002-203422, and Japanese Kokai Publication No. 2002-350846). For example, Japanese Kokai Publication No. 2001-43718 discloses the following light unit. A light source portion in a light guide plate is provided with a reflection and refraction portion and thereby part of the light, with a high luminance near the center, emitted from a light emitting diode (LED), is not directly guided into an output portion, but into both side surfaces. Further, irregular reflection parts on both side surfaces are arranged in such a way that an area ratio of the irregular reflection portion increases as it is further from the light sources, and thereby a light amount near the light sources is suppressed and lights from the LED light sources are uniformly output from an output surface with efficiency. Japanese Kokai Publication No. 2002-100231 discloses that point light sources are arranged at a pair of opposing corners of a light guide plate, and a light guide portion having a light-entering surface and a reflection surface is formed integrally with the light guide plate so as to protrude outwardly. As a result, light from the light sources is spread over the entire light guide plate, and generation of uneven luminance is prevented. However, even if these technologies are used, a dark portion where the luminance is low is generated at a position far from the light source or at a corner portion of the display region, or the backlight or the frontlight might have a complicated structure. In such points, there is room for improvement.
In the side-light type backlight or frontlight, a cold cathode fluorescent lamp (CCFL) that is a linear light source or an LED that is a point light source, and the like are used as a light source. Recently, LEDs tend to be increasingly used because they can be driven at a low voltage. For example, an LED has been used in a display of a portable apparatus such as a cellular phone. However, if a light source which emits light with a strong straight forward property, such as an LED, is used, uneven luminance and the like tends to be generated. In this point, there is room for improvement. For example, if an LED is used as the light source, a sidelight type backlight or front light includes, as shown in FIG. 15A, a rectangular light guide plate 201 and a plurality of light sources 203 arranged in a line to oppose one side surface on the side of a long side of four sides of the rectangular light guide plate 201. As shown in FIG. 15B, light sources 203a at both terminals of these light sources 203 are arranged in a region opposing an end portion 102a of the display region 102. However, according to such a light-irradiating method, an end portion 102a of the display region, particularly at a corner portion 102b is not sufficiently irradiated with light in a sufficient amount. Therefore, this region becomes a dark portion where the luminance is lower than that in other display regions. As a result, display uniformity is deteriorated. FIG. 15B schematically shows overlapping of lights from the respective light sources. FIG. 15C shows measurement results of a luminance distribution. As shown in FIGS. 15B and 15C, a dark portion is generated at the end portion 102a of the display region, particularly at a corner portion 102b. The luminance uniformity in the entire display region 102 is deteriorated.
Under such circumstances, a light unit which can obtain sufficient luminance at the end portion of the display region and which can radiate light having a high luminance and excellent uniformity to the entire display region, without a complicated structure, has been needed.